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Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have attracted significant interest in various optoelectronic applications due to their excellent nonlinear optical properties. One of the most important applications of TMDs is to be employed as an extraordinary optical modulation material (e.g., the saturable absorber (SA)) in ultrafast photonics. The main challenge arises while embedding TMDs into fiber laser systems to generate ultrafast pulse trains and thus constraints their practical applications. Herein, few-layered WS with a large-area was directly transferred on the facet of the pigtail and acted as a SA for erbium-doped fiber laser (EDFL) systems. In our study, WS SA exhibited remarkable nonlinear optical properties (e.g., modulation depth of 15.1% and saturable intensity of 157.6 MW cm) and was used for ultrafast pulse generation. The soliton pulses with remarkable performances (e.g., ultrashort pulse duration of 1.49 ps, high stability of 71.8 dB, and large pulse average output power of 62.5 mW) could be obtained in a telecommunication band. To the best of our knowledge, the average output power of the mode-locked pulse trains is the highest by employing TMD materials in fiber laser systems. These results indicate that atomically large-area WS could be used as excellent optical modulation materials in ultrafast photonics.

We report the fabrication process of isolated one-dimensional Se chains incorporated in the matrix of AlPO4-5 single crystals and the experimental investigation of the geometry, phase stability, electronic properties, and electron-phonon coupling effect of these Se chains. The structure of the helical Se chains inside the channels is discussed on the basis of X-ray scattering measurements. Thermal analysis and temperature-dependent micro-Raman measurements show that Se single chains are flexible and can convert from a weak distorted phase into another phase with strongly disordered structure ("melting" state) around 340 K. Since the electrons are confined in the one-dimensional channels, the absorption band of the Se chain is obviously blue shifted compared with that of trigonal Se. With increasing temperature, this band shifts linearly to the lower energy side, in sharp contrast to the nonlinear temperature coefficient of trigonal Se, which is attributed to the greatly diminished interchain interaction and the weakening of the electron-optical phonon coupling in a low-dimensional system. In the vicinity of the absorption band, both first-order and second-order Raman signals for the Se chain are enhanced, due to the strong electron-phonon coupling when the excitation laser energy matches the electronic transition in isolated Se chains.